I think holding WC in Brazil is a bad idea because many top cubers won't be there and this, as other people have already pointed out, will make the WC title less valuable.
Holding the WC in Brazil will motivate more of the Brazilian and South American cubers who are close to world class to practice to become world class. This increases the quality of competition worldwide.
What does it mean to be the world champion for, lets say, 3BLD, if you can't even get to podium in a random polish competition?
The title World Champion does not mean "best in the world" in our community, it means "the person who won the final round of an event at that year's world championship".
I can use your argument to say that every world champion's title is watered down, because in all cases there surely was at least one cuber who was world class or near to it who could have gotten lucky solves or had stronger nerves than the person who did win.
Using your argument, I will define a world champion as being a "pure" world champion only when they win at worlds and every single competitor in the WCA database was present and competed in that event. Even that is not fully "pure" because of the people in the world who know how to solve the cube who have never even heard of the WCA were not present. So even that world champion's title is "less valuable" to use your terminology.
Take your argument to its extreme and you will see that every world title is "less valuable" in some respect.
Every choice of location for the World Championship in the past has made some people angry, and every world championship has had world class cubers who could not attend. This is what happens when you host world competitions, and it is an inherent part of our sport. This choice of venue does a lot of good for our community in that it will have a significant increase in popularity of cubing in South America, as well as in the world class quality of the south american cubers. By your definition this means that some of the world champion titles may be "less valuable" this time around (I don't think so, but you seem to), but that will also make the level of competition much higher when the newly world class South American cubers travel to worlds in 2017, 2019, etc..
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tl;dr
Take the long term view, not the short term view.
I think that the overall benefit to the cubing community of hosting worlds in South America in 2015 is far greater than any "loss in value" of any particular world champion title in 2015.
Using your criteria, every world champion title ever awarded is "less valuable"
